Before using threaded precision parts, it is necessary to check various characteristics of the threads such as, for example, lead error, pitch diameter, and major diameter. As is well known, "lead" is the axial distance between the centers of adjacent grooves, and "lead error" is the variance between the actual lead and the true lead, i.e., the lead of perfect threads. If the lead error is too great, the threads are unacceptable.
Lead error is difficult to check or measure. Some prior art gages check lead error directly. Unfortunately, however, these gages are expensive and complex. Other prior art gages do not check lead or lead error directly. Rather, an effort to check lead error is made by determining whether or not certain specially designed thread elements seat to a proper depth in the threads.
One problem with this technique is that a failure of the threaded element to seat a proper depth in the threads may be caused by thread characteristics other than lead error. Thus, this prior art technique may indicate that the lead error is outside acceptable tolerances when, in fact, the lead error is within those tolerances.
Conversely, compensating errors may cause the threaded elements to seat an acceptable depth in the threads even though the lead error may be outside the allowable tolerances. Thus, with prior art techniques, compensating errors can mask or partially mask lead error.